Actually exMARS is a redcode simulator, just like exhaust and pMARS.
In fact, I have shamelessly taken sourcecode from pMARS, exhaust, some
ideas from qMars, a shot of optimizations, shook everything well, and
garnished everything with a high level interface for
Ruby.

The resulting program has the following main features:

Uses the parser from pMARS, so no previous parsing is neccessary.
At first this was my main motivation for exMARS.

Speed: 50% faster than pmars on a pentiumIII, and often more than
twice as fast than pmars pentium 4 (using
gcc 3.3.1, and the same
compiler options).

Rewritten the code in a more object oriented way, which allows
different Mars at the same time in the same program, it should also
be thread safe.

Ruby interface: finally
a really fast mars can be used in a high level programming language.
(see test.rb for an example usage)

At the moment exmars is probably an alpha version. It contains almost
all features I want it to have, but the programming interface will
definitely change.

I have only tried exMARS in Linux, because I do not have access to
windows right now, a simple 'make' would do. Have a look at the
Makefile for optimizations.